A color film recorder is used to convert computer generated graphics, typically viewed on a cathode ray tube (CRT) display screen, into photographic slides and prints. The computer generated image is first displayed on a CRT display screen within the recorder and the image is then captured on film. The quality and resolution of the final slide or print depends critically upon the resolution with which images are formed on the CRT.
The modern CRT display screen may be modeled with a two dimensional N.times.N grid such that resolution is directly related to the density of the grid. The prior art offers two types of CRT display screens, raster displays and analog vector displays. In raster displays, the display screen consists of a fixed grid of pixels and images are formed by selective illumination of individual pixels. Typically, the luminous intensity of each pixel may also be varied. In analog vector displays, the grid consists of addressable locations on a voltage grid. Such locations may be addressed with digital signals which are generated by a microprocessor and converted to voltage levels with digital-to-analog (D/A) converters. Images are formed by lines drawn with the smooth, continuous sweep of the CRT electron beam between locations on the voltage grid.
Although raster displays enjoy an increasing commercial dominance in CRTs, they do have an inherent drawback which is particularly harmful to image quality in film recorders. Since raster images consist of individual pixels, such images tend to exhibit jaggedness, particularly along lines which are nonparallel to the principal axes of the raster display. A partial solution to jaggedness is increased pixel density but such an increase requires costly additional hardware in the form of additional pixels and possibly additional control electronics. Alternatively, the appearance of jaggedness maybe reduced by using a technique known as raster anti-aliasing. Raster anti-aliasing involves balancing the intensity of the overall image by varying the intensity of individual pixels. The net effect is a partial smoothing of the jagged edges. This technique, however, is computationally very expensive in raster displays. Increasing the effectiveness of raster anti-aliasing may also involve a significant hardware cost, particularly in the electronics used to control pixel intensity. Unfortunately, the hardware costs of increased pixel density and the computational costs of raster anti-aliasing may become unacceptable at the resolution level necessary to produce high-quality film recording.
Analog vector displays do not have the inherent jaggedness problem of raster displays. In fact, since lines in the analog display are formed by the smooth, continuous sweep of the electron beam from one addressable location to another, such lines have essentially infinite resolution between endpoints. Therefore, resolution in analog vector displays depends solely upon how precisely the endpoints of lines may be positioned, that is, solely upon the density of the addressable locations in the voltage grid. Density of the voltage grid is a function of the size of the D/As which produce voltage levels. Each doubling of the grid density requires incrementing the bit length of the D/As. Unfortunately, the hardware costs become prohibitive at the higher grid densities required for quality film recording as larger D/As become expensive or unavailable.